The day after they met at a party, the man drove the girl home, let her take a shower and gave her some fresh clothes. Then he told her, “We’re going to work.”

At first, the girl didn’t understand what the man meant. But everything became horribly clear after they pulled into a pharmacy parking lot and she watched another gang member return to the car with a box of condoms. The next stop was her first customer.

For three months, the girl was prostituted almost daily in dingy apartments, motels and even at an auto repair shop. The men paid $40 for 15 minutes of sex.

Disturbing in itself, the case also illustrates what authorities say is an emerging and troubling trend — the girl was not alone. The 12-year-old was one of dozens of prostitutes, many juveniles, being sold for sex in the Washington area by members of Mara Salvatrucha, also known as MS-13, the region’s largest and most dangerous street gang.

Jose Ciro Juarez-Santamaria (Courtesy of US Dept. of Justice/COURTESY OF US DEPT. OF JUSTICE)

In recent months, authorities have arrested four alleged MS-13 members or associates in Northern Virginia on federal prostitution-related charges involving juvenile victims. Three have been convicted. Last month, a federal judge sentenced the man who prostituted the 12-year-old to life in prison, calling it “the most difficult case I have ever listened to.”

In recent years, authorities say, MS-13 in the D.C. area has largely stepped back from the violent attacks that helped cement its national reputation as a street gang whose motto, “rape, control, kill,” instilled fear in many communities. Although the gang continues to engage in illegal activity ranging from extortion to drug dealing, the recent arrests signal that it is working to expand its criminal enterprises.

Mara Salvatrucha, which has 1,500 to 3,000 local members, took root in Los Angeles in the mid-1980s with men from El Salvador who had fled the violent civil war in their country.

The gang came to prominence in the Washington region after a series of brutal attacks in the last decade. MS-13 members fatally stabbed a pregnant teenager after learning she was a federal witness and used a machete to sever four fingers from a 16-year-old boy’s hand. The gang has been linked to shootings and baseball bat beatings.

That violence prompted a crackdown by federal and local authorities that resulted in convictions of dozens of MS-13 members for crimes ranging from drug dealing to murder. The power vacuum that was created when MS-13 leaders were locked up has prompted remaining members to scramble to find ways to make money, said Joshua Skule, an assistant special agent in charge for the FBI’s Washington Field Office.

“If they can prostitute juveniles and not get a whole lot of fanfare, then they are going to try to go that way,” Skule said, adding that agents are spending “significant time with victims to build cases” against MS-13 members.

In court papers and interviews, federal authorities described in detail how the four MS-13 members operated three sex-trafficking rings.

With nicknames such as Murder, Sniper and Casper, the gang members advertised the rings using business cards for “towing” companies. When calls came in — one member’s “phone rang constantly with clients calling for prostitution appointments,” according to the FBI — they drove the girls to meet clients. One pimp even set up shop in his mother’s home.

To ensure that they got paid and that customers did not become violent with the girls, they brought weapons; one MS-13 member carried a machete, one of the gang’s most notorious and symbolic weapons, court papers show.

Depending on the prostitute’s age — younger girls commanded a premium — the men charged $40 to $100 for 15 to 30 minutes of sex. At least one MS-13 member offered discounts to customers who brought more men.

The gang members also had sex with the girls, whom they met at schools, parties or through friends. Most of the victims were runaways who felt they had nowhere to turn for help. One 15-year-old told authorities that she met an MS-13 member after a middle school dance, FBI agents wrote in court papers, and a 14-year-old runaway told authorities that she met her pimp in a park.

To keep them compliant, the MS-13 members gave the girls liquor and marijuana before they sold them for sex. The gang took most of the cash and gave the girls food and a place to stay.

“The very people they are looking to for help end up taking advantage of them,” Torres said. “There are physical and psychological repercussions that can take years for them to overcome.”

That’s how it was for the 12-year-old girl, the youngest victim whom authorities have discovered. (The Washington Post generally does not identify victims of sexual crimes).

Speaking softly on the witness stand in federal court in Alexandria during the July trial of her MS-13 pimp, Jose Ciro Juarez-Santamaria, the now-14-year-old described her path from normal kid to child prostitute. It started, she said, when she was 11 and ran away from her father in Maryland to live with her boyfriend. Kicked out by his parents, she spent a few months moving “from house to house, basically living in the streets.”

Soon she was going to parties thrown by MS-13 associates. It was at a Halloween party in Oxon Hill that she met and sought the help of Juarez-Santamaria, who goes by the nickname Sniper. At some point, she told him she was 17.

The next day, he took her to his sister’s apartment, where she got a shower and clothes. Within a few hours, she was having sex with at least four men on a dirty mattress in an auto repair shop.

Her life continued that way for months.

After sex, the men would hand over cash, and she would turn it over to Juarez-Santamaria and other gang members. But she saw little money herself. Occasionally, prosecutors say, he gave her some cash for food. She had no home and stayed at motels or the apartments of MS-13 associates.

To steel herself for work, she drank vodka and smoked marijuana. “I felt uncomfortable being sober,” the girl said. “I just did. I didn’t feel normal.”

By December, someone had noticed a missing person’s flier displaying the girl’s real age and photograph and took it to an Oxon Hill apartment where they were all staying, dispelling any notion that the prostitute was a teenager.

But the truth didn’t bother Juarez-Santamaria, prosecutors say.

He just put the girl back to work.

A few days later, acting on a tip, federal agents raided the apartment and rescued the 12-year-old. But she ran away from a shelter and returned to Juarez-Santamaria. She would be picked up again and run away again — a pattern she repeated a number of times until she finally found a safe place to stay. During it all, the girl lied repeatedly to investigators, denying ever having been a prostitute.

At the trial, a prosecutor gently asked why the girl hadn’t been more honest: Was it because she thought the MS-13 members were her friends?

“I guess because I had nobody else,” she answered, adding that she had “always heard that if you snitch you get killed.”